Thermal considerations:
At continuous current, the voltage across the emitter-base junction VBE of a bipolar transistor get decreases 2 mV (silicon) and 1.8mV (germanium) for every 1°C rise in temperature (reference being 25°C). Through the Ebers-Moll model, if the base-emitter voltage VBE that is held constant and the temperature increases, the current by the base-emitter diode IB will raise, and so the collector current IC will as well increase. Depending upon the bias point, power dissipated in the transistor might as well increase that will further increase its temperature and exacerbate the problem. This deleterious positive (+ive) feedback results in thermal runaway. There are various approaches to mitigate bipolar transistor thermal runaway. For instance,
1. Negative feedback can be made into the biasing circuit so that increased collector current directs to decrease base current. Therefore, the increasing collector current throttles its source.
2. Heat sinks can be employed which carry away extra heat and prevent the base-emitter temperature from rising.
3. The transistor can be biased that is why its collector is usually less than half of the power supply voltage that implies that collector-emitter power dissipation is at the maximum value of it. After that runaway is impossible because increasing collector current directs to a decrease in dissipated power; this notion is termed as the half-voltage principle.
The circuits below primarily illustrate the use of negative feedback to avoid thermal runaway.